**Null Hypothesis:** A null hypothesis is a statistical concept that assumes there is no significant difference or relation between certain aspects of a study or experiment. In other words, it's the hypothesis that your test is aiming to disprove. For example, in an A/B test, the null hypothesis might be that there's no difference in conversion rates between version A and version B of a webpage. If the test results show a significant difference, then you can reject the null hypothesis.